feat(cleanup): walk every backend, reap smolmachines orphans too

`./cli.py cleanup` previously called only the env-var-selected
backend's `prepare_cleanup` / `cleanup` — so a leftover smolvm
machine + bundle container + bundle network from a crashed
smolmachines bottle would survive a default `docker`-mode cleanup
indefinitely.

Smolmachines now has a real `cleanup` module (alongside
`enumerate.py` from issue #77) that walks:

  - smolvm machines named `claude-bottle-*` (via
    `smolvm machine ls --json`)
  - bundle containers `claude-bottle-sidecars-*`
  - bundle networks `claude-bottle-bundle-*`

Cleanup runs stop+delete on the machines, force-rm on the
containers, network rm on the networks. Each step is best-effort
so a failed rm doesn't block the rest.

`cli.py cleanup` walks every backend in `known_backend_names()`
and runs each backend's `cleanup` after a single y/N prompt that
shows a combined plan.

State dirs (`~/.claude-bottle/state/<slug>/`) are shared layout
with the docker backend, which still owns the orphan-state-dir
bucket. It now consults `enumerate_active_bottles()` for the
cross-backend live identity set so a running smolmachines
bottle's state dir isn't reaped during a cleanup.

Tests: smolmachines cleanup (prepare + cleanup ordering + failure
handling); cross-backend orphan protection on the docker
state-dir check; CLI cmd_cleanup walks both backends, short-
circuits on all-empty, aborts on N. 617 unit tests pass.

End-to-end verified on this host:
  $ smolvm machine ls --json | jq '.[].name'
  "claude-bottle-researcher-m3hxd"
  $ ./cli.py cleanup
  --- smolmachines backend ---
  smolvm machine:  claude-bottle-researcher-m3hxd
  remove all of the above? [y/N]

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/smolmachines/backend.py

@@ -8,6 +8,7 @@ from pathlib import Path
 from typing import Generator, Sequence
 
 from .. import ActiveAgent, BottleBackend, BottleSpec
+from . import cleanup as _cleanup
 from . import enumerate as _enumerate
 from . import launch as _launch
 from . import prepare as _prepare
@@ -76,12 +77,10 @@ class SmolmachinesBottleBackend(
         _supervise.provision_supervise(plan, target)
 
     def prepare_cleanup(self) -> SmolmachinesBottleCleanupPlan:
-        return SmolmachinesBottleCleanupPlan()
+        return _cleanup.prepare_cleanup()
 
     def cleanup(self, plan: SmolmachinesBottleCleanupPlan) -> None:
-        del plan
-        # Nothing to clean in chunks 1-3 — see
-        # SmolmachinesBottleCleanupPlan docstring.
+        _cleanup.cleanup(plan)
 
     def enumerate_active(self) -> Sequence[ActiveAgent]:
         return _enumerate.enumerate_active()